In the production of integrated circuits, a mask or reticle is used in conjunction with a radiation source and lens system to define the desired pattern of circuit features and interconnections on a wafer or substrate of semiconductor material. A mask is used to transfer a pattern onto an entire wafer in a single exposure step, while a reticle is used to transfer multiple exposures of the same pattern onto a wafer in "step-and-repeat" fashion.
A reticle is composed of a transparent substrate on which is deposited a thin layer of material that serves as the template for the pattern which will be transferred to the wafer surface. The substrate material is typically soda-lime glass, borosilicate glass, or quartz. The material with which the glass is coated is typically emulsion, chrome, or iron oxide and is deposited on the glass by means of sputtering or evaporation techniques.
The material deposited on the glass serves as the pattern template and is etched to contain a multitude of apertures which, by their shapes, sizes, and locations, define the structure of the device which is to be fabricated. The template features are then defined on a photoresist coated wafer of semiconductor material by means of known photolithographic processes.
The first step in the reticle making process is to "write" the pattern of features onto the chrome layer (or other pattern material) of the reticle. This defines those regions of the material which will be removed to form the pattern template. The reticle is coated with a thin layer of photoresist, or if an E-beam (electron beam) is used to write the patterns, a suitable E-beam resist. The reticle is then exposed by an optical or E-beam system. The exposure defines the desired patterns on the resist layer. Two types of scanning tools are presently used to write the device patterns. They are termed "raster" scanners (which scan the beam in the y-direction while the reticle is moved in the x-direction), and "vector" scanners (which scan the beam in both the x and y directions over the reticle).
The patterns defined on the resist layer are generated in the form of rectangles. Each rectangle is defined by a height (H), width (W), x and y position coordinates, and an angle (.theta.) representing the orientation of the rectangle relative to the coordinate axes. The writing tool exposes the resist covered pattern material while it is moved under the tool by a stage whose position is controlled by a laser interferometer. After the writing step is completed, the resist is developed and then baked prior to the etching step. The etching step removes portions of the chrome or other pattern material, resulting in the finished pattern template.
After the reticle is fabricated, it is used in conjunction with a projection system to define the device features on a resist coated wafer of semiconductor material. In defining the device features on the wafer, a lens is used to project an image of the reticle onto the wafer. A lens may also be used to reduce the mask image at the wafer surface so that multiple mask images can be defined on the resist layer.
Another method of defining device features on a wafer is to write the device features directly onto the wafer without the use of a mask or reticle. This method uses an electron beam or optical source which is transmitted through a projection system and which directly exposes the resist coated wafer. A direct writing tool also uses a controlled stage to position the wafer beneath the tool.
A problem that can arise when using either of these techniques is that a lens may have inherent errors which distort light passing through it. Standard types of lens field errors are trapezoid errors, anamorphisms, and linear or non-linear reduction errors. A common type of error is that which arises when the magnification of the lens varies with the radial distance from the lens center. This makes the magnification power of the lens a function of the region on the lens surface through which the source radiation passes. This type of lens field error leads to the "pincushion" or "barrel" effect found in some images.
Lens field errors introduce manufacturing flaws into the integrated circuit fabrication process because the exposed regions of the resist layer do not represent an exact image of the template. When mixing different types of exposure tools in a manufacturing process, errors can occur because each tool, or set of tools, has its own distortion pattern. This means that some layer geometries that are patterned by one type of tool, may be displaced relative to previous layer(s) that were patterned by a different tool. In the case of a direct writing method, patterns written by these tools will not perfectly overlay previous layers patterned by optical tools using lenses with inherent distortions. Lens errors can lead to the incorrect placement of features relative to each other, and to a reduced yield of correctly operating devices.
A method of compensating for lens distortions when using a vector scanner as a direct writing tool has been developed by IBM (International Business Machines), and is described in P. Coane, et al., "Electron Beam/Optical Mixed Lithography at Half-Micron Ground Rules", in Microelectronic Engineering 5 (1986), pp. 133-140, published by Elsevier Science Publishers B.V. A similar approach is described in T. Newman, et al., "High Resolution Shaped Electron Beam Lithography", in Microlithography World, March/April 1992, p.16.
The method discussed in these references is based on dividing the field of the lens into sub-fields and then calculating the distortion correction for the center point of each sub-field. The correction for the center points is then applied to all points in the sub-field by means of registration marks placed on the periphery of the exposure field. The marks serve as alignment guides for the tool when it exposes the resist coated wafer during the fabrication of the device. The placement of the marks is a function of the lens distortion data, and results in a uniform shift in the location of the features. This method compensates for the lens errors by altering the placement of the device features on the wafer as the resist layer on the wafer is exposed.
The above method is limited to fabrication processes which use direct writing tools, and its effectiveness is limited by the need for alignment structures on the periphery of each field and the amount of data storage available. This is because, in order to compensate for higher order distortion terms, the scale on which the corrections are applied must be smaller. Thus, the lens field must be divided into a greater number of sub-fields so that the correction applied to each center point is sufficient to compensate for the distortion. This requires that there be enough storage capacity to store the distortion data for the lens, and the corresponding correction terms used to remove the distortion.
Another method of correcting for lens distortions when using vector E-beam writing tools is based on the proximity correction software which is part of such tools. E-beam vector scanners utilize the software to correct for the effects of scattering of the electron beam in the resist material by adjusting the exposure times of adjacent features. This compensates for the exposure of a feature adjacent to the one being written due to the scattering of electrons out of the main beam. The proximity correction software can be used to compensate for lens distortions by scanning the device feature after it has been exposed using the stepper lens, and using the results as the basis for corrections which are fedback into the writing tool as though they were proximity errors, for use when the same feature is subsequently written. This correction scheme is limited to use with vector scanners because it relies on the writing tool having access to the coordinates of each point so that the corrections can be applied to each point as it is written.
What is desired is a means of compensating for lens field errors when using a raster or vector scanner to fabricate a mask or reticle, where the means does not require a large amount of data storage memory, and where the means is capable of correcting for the field errors independently of the grid size of the mask making or device fabrication tool.